SUMMARY Although improvements in cancer screening and treatment have decreased patient mortality for many of the nearly 1.7 million new cases diagnosed annually, survivors increasingly experience the debilitating side effects of ionizing radiation therapy (RT). For example, the percentage of femoral neck and pelvic fractures are elevated in postmenopausal women irradiated for cervical (+66%), rectal (+65%), and anal (+216%) cancers by five years post-RT. An early increase in active bone resorption by osteoclasts appears to occur after RT; this acute bone loss persists and likely leads to fractures. However, no interventions currently exist because of the toxicities associated with antiresorptive therapies and RT, including osteoradionecrosis. This project will determine if the novel compound, 48c, is a well-tolerated, translatable intervention that can prevent radiation- induced osteoclast activity, differentiation, and bone loss. 48c inhibits the activity of inositol requiring ER-to- nucleus signal kinase-1 (IRE-1?), which when activated in osteoclasts enhances differentiation and activity. Inhibiting IRE-1? lowers tumor growth and invasiveness, and thus, is an anticancer strategy. In Specific Aim 1, molecular, cytologic and resorption assays will be used to determine if pretreatment with 48c prevents RT- induced osteoclast differentiation and activity in vitro by inhibiting IRE-1? signaling in osteoclast precursors. The studies will be performed on irradiated RAW 264.7 cells and primary osteoclast precursors isolated from both wild-type and IRE-1? deficient C57BL/6 mice. In Specific Aim 2, microCT, histologic, and biochemical assays will be used to determine if IRE-1? inhibition prevents or ameliorates early (2 weeks) and late (12 week) RT-induced bone loss, architectural deterioration, and loss of strength in wild type C57BL/6 mice that received subcutaneous injections of 48c prior to RT and IRE-1? deficient C57BL/6 mice. Successful completion of these Specific Aims will determine if 48c is a well-tolerated, translatable intervention to prevent the debilitating RT-induced pelvic, vertebral, and hip fractures in cancer survivors treated with RT.